Photo-to-photo conversion element and its applied system

ABSTRACT

A photo-to-photo conversion element is disclosed. The element has, in an arranged sequence in line: a first optical member having a wavelength selectivity permitting passage of a write light having a first wavelength range and blocking an erase light; a first transparent electrode; a photoconductive layer; a second optical member having a wavelength selectivity permitting reflection of a read light having a second wavelength range and passage of the erase light; a third optical member for electrooptically modulating a light incident thereto through the second optical member in response to the strength distribution of an electric field applied thereto; and a second transparent electrode. The erase light has a wavelength outside of both the first and second wavelength ranges.

This application is a divisional application of Ser. No. 313,073 filedFeb. 21, 1989, now U.S. Pat. No. 4,920,417, which was acontinuation-in-part application of Ser. No. 139,005 filed Dec. 29,1987, now U.S. Pat. No. 4,831,452.

BACKGROUND OF THE INVENTION

This invention relates to a photo-to-photo conversion element suitablefor image pickup devices, optical writing projectors, and the like.

Image pickup devices using photo-to-photo conversion elements are freefrom problems encountered with conventional image pickup devices, andare therefore able to generate with ease a video signal for providing areproduced pictorial image having high picture quality and highresolution.

An example of such an image pickup device constituted withphoto-to-photo elements can be readily understood by making reference toJapanese Patent Application No. 311333/86 entitled "Image pickup device"filed on Dec. 30, 1986 by the assignee.

For a photo-to-photo conversion element having a structure capable ofreceiving an optical image and outputting an optical image also as itsoutput, attention has been conventionally paid to elements, e.g., aliquid crystal optical modulator, a photoconductive Pockels cell, aspatial light element such as a microchannel light modulator, an elementconstituted by a photochromic material and the like as, e.g., an opticalwriting projector, an element for optical parallel processing of anoptical computer, an element for recording a picture, etc. In addition,the assignee has also proposed an image pickup device having highresolution using photo-to-photo conversion elements.

FIG. 1 is a side cross-sectional view showing an example of theconfiguration of a conventional photo-to-photo element. Thephoto-to-photo conversion element shown in FIG. 1 has an arrangementcomprising, in a stacked sequence, a glass plate 1, a transparentelectrode 3, a photoconductive layer 7, a light screening or shieldinglayer 12, a dielectric mirror 8, an optical member (e.g., an opticalmodulation layer such as lithium niobate monocrystal or a nematic liquidcrystal layer) 9 for changing the state of light in dependence upon anapplied electric field, a transparent electrode 4, and a glass plate 2.This photo-to-photo conversion element is such that a write light WL, aread light RL, and an erase light EL are irradiated from the glass plate1 side, the glass plate 2 side, and the glass plate 1 side,respectively.

In the photo-to-photo conversion element shown in FIG. 1, a circuitcomprising a power supply 10 and a changeover switch SW is connectedbetween terminals 5 and 6. By a switching control signal delivered to aninput terminal 11 for the switching control signal in the changeoverswitch SW, the movable contact of the changeover switch SW is switchedto the fixed contact WR side. Under this condition, a voltage of thepower supply 10 is applied across the transparent electrodes 3 and 4, totherefore apply an electric field across both terminals of the opticalmember 9. Furthermore, when the write light WL is caused to be incidentfrom the glass plate 1 side in the photo-to-photo conversion element totransmit the incident write light WL through the glass plate 1 and thetransparent electrode 3 to reach the photoconductive layer 7, becausethe electric resistance value of the photoconductive layer 7 changes incorrespondence to an optical image of the incident light which hasreached the photoconductive layer 7, a charge image, corresponding tothe optical image of the incident light which has reached thephotoconductive layer 7, is produced at the boundary surface between thephotoconductive layer 7 and the light screening layer 12.

Furthermore, under the condition where the movable contact of thechangeover switch SW is switched to the fixed contact WR side aspreviously described, an electric field having a strength distributioncorresponding to the charge image, having been produced by the writelight as described above at the boundary surface between thephotoconductive layer 7 and the light screening layer 12, is applied tothe optical modulation layer 9 such as a lithium niobate monocrystal (ornematic liquid crystal layer) 9 provided so as to have a serialrelationship with respect to the above-mentioned photoconductive layer 7along with the light screening layer 12 and the dielectric mirror 8,etc. between transparent electrodes 1 and 2 across which a voltage ofthe power supply 10 is applied through terminals 5 and 6. Thus, the readlight RL incident from the glass plate 2 side changes to a reflectedlight including pictorial image information corresponding to thestrength of an electric field applied to the optical modulation layer 9by the electro-optic effect of the optical modulation layer 9, and isthus emitted from the glass plate 2 side.

The light not reflected by the dielectric mirror 8 of the read lightincident to the glass plate 2 side, as stated above, goes on through apath including the transparent electrode 4, the optical modulation layer9, the dielectric mirror 8 and the light screening layer 12, and isscreened by the light screening layer 12 so that it does not go furthertoward the photoconductive layer 7 side. For this reason, even if theread light RL is incident or projected to the glass plate 2 side, thereis no possibility that the electric resistance value of thephotoconductive layer 7 changes thereby. Thus, even read light RLincident to the photo-to-photo conversion element gives no possibilityof a change in the charge image produced in correspondence with theoptical image by the incident light at the boundary surface between thephotoconductive layer 7 and the light screening layer 12.

On the other hand, in the above-mentioned photo-to-photo conversionelement shown in FIG. 1, the information written thereinto by the writelight WL will be erased as follows. First, a switching control signal isdelivered to the input terminal 11 for the switching control signal inthe changeover switch SW to switch the movable contact of the chageoverswitch SW to the fixed contact E side, to therefore allow the terminals5 and 6 to have the same potential so that no electric field is producedacross the transparent electrodes 3 and 4. Then, by allowing an eraselight EL having a uniform strength distribution to be incident from theglass plate 1 side which is the incident side of the write light WL, theerase light EL is given to the photoconductive layer 7 through the glassplate 1 and the transparent electrode 3, thus placing thephotoconductive layer 7 in the state where its electric resistance valueis lowered to erase a charge image produced at the boundary surfacebetween the photoconductive layer 7 and the light screening layer 12.

The reason why the side on which the erase light is incident for erasingthe information having been already written into the conventionalphoto-to-photo conversion element shown in FIG. 1 stated above, is thesame as side on which the write light WL in incident is as follows.Since there is the light screening layer 12 between the side of whichthe read light RL and the photoconductive layer 7, even if the eraselight is caused to be some from the incident side on which the readlight RL is incident, that erase light is stopped by the light screeninglayer 12, and therefore failing to reach the photoconductive layer 7.Accordingly, even if the erase light is caused to be same from theincident side on which the read light RL is incident, a charge imageoccurring at the boundary surface between the photoconductive layer 7and the light screening layer 12 cannot be erased.

This is a serious problem when the photo-to-photo element is used, e.g.,in an image pickup device of a structure such that it is required toprovide an image pickup optical system on the side on which the writelight WL is incident, or in a device of a structure such that it isdifficult to provide the incident unit for the erase light on the sideon which the write light WL is incident.

For a photo-to-photo conversion element which can solve theabove-mentioned problem, the assignee has already proposed aphoto-to-photo conversion element constructed as shown in FIG. 2, i.e.,a photo-to-photo conversion element comprising, in a stacked sequence, aglass plate 1, a transparent electrode 3, a photoconductive layer 7, anoptical member 8R having wavelength selectivity permitting a light inthe wavelength region of the read light to be reflected and permittinglight in the wavelength region of the erase light to be transmitted, anoptical member 9 for changing the state of light in dependence upon anapplied electric field, a transparent electrode 4, and a glass plate 2.

For such an optical member 8R, e.g., a member constituted by a dichroicfilter comprised of a multilayer layer consisting of a thin film of SiO₂and a thin plate of TiO₂ may be used.

Moreover, for the optical member 9, e.g., an electro-optic effectcrystal such as a lithium niobate monocrystal, or an optical memberconstituted by a nematic liquid crystal layer may be used. In FIG. 2,WL, RL and EL denote a write light, a read light and an erase light,respectively.

FIGS. 3A to 3D are characteristic curves showing the wavelengthselection characteristic of the optical member 8R having a wavelengthselectivity permitting a light in the wavelength region or band of theread light to be reflected and permitting a light in the wavelengthregion of the erase light to be transmitted, respectively. FIG. 3 showsthat the optical member 8R having a characteristic shown in FIG. 3A isconstituted as an optical low-pass filter, the optical member 8R havinga characteristic shown in FIG. 3B is constituted as an optical high-passfilter, the optical member 8R having a characteristic shown in FIG. 3Cis constituted as an optical band-pass filter, and the optical member 8Rhaving a characteristic shown in FIG. 3D is constituted as an opticalband-rejection filter.

Namely, in the photo-to-photo conversion element previously proposedwhich is shown in FIG. 2, for the optical member 8R used as a part ofthe components thereof, i.e., the optical member 8R permitting light inthe wavelength region of the read light to be reflected and permittinglight in the wavelength region of the erase right to be transmitted,optical members 8R having such wavelength selectivities as shown inFIGS. 3A to 3D may be used.

In the photo-to-photo conversion element previously proposed, which isprovided with the optical member 8R capable of arbitrarily having anyone of the wavelength selectivities as shown in FIGS. 3A to 3D, light ina wavelength region where the transmission factor of light is low in theoptical member 8R, is used as the read light to be incident to thatphoto-to-photo conversion element, and light in a wavelength regionwhere the transmission factor of light is high in the optical member 8R,is used as the erase light to be incident to the photo-to-photoconversion element. Thus, the photo-to-photo conversion elementpreviously proposed permits an erase light to be same from the incidentside on which the read light is incident.

In the case of writing optical information into the photo-to-photoconversion element previously proposed which has the arrangement shownin FIG. 2, a circuit composed of power supply 10 and changeover switchSW is connected between terminals 5 and 6 of the photo-to-photoconversion element to allow the movable contact of the changeover switchSW to be switched to the fixed contact WR side by a switching controlsignal delivered to the input terminal 11 for the changeover controlsignal in the changeover switch SW. Under this condition, a voltage ofthe power supply 10 is applied across the transparent electrodes 3 and4, to therefore apply an electric field across both terminals of thephotoconductive layer 7. Furthermore, when the write light WL isincident from the glass plate 1 side in the photo-to-photo conversionelement, writing of the optical information into the photo-to-photoconversion element will be conducted as follows.

Namely, when the write light WL incident to the photo-to-photoconversion element is transmitted through the glass plate 1 and thetransparent electrode 3 to reach the photoconductive layer 7, becausethe electric resistance value of the photoconductive layer 7 changes incorrespondence to an optical image of the incident light which hasreached the photoconductive layer 7, a charge image corresponding to theoptical image of the incident light which has reached thephotoconductive layer 7 is produced at the boundary surface between thephotoconductive layer 7 and the optical member 8R.

In order to reproduce, from the photo-to-photo conversion element, theoptical information which has been written in the form of a charge imagein correspondence with the optical image of the incident light in amanner as stated above, there may be employed a method to switch themovable contact of the changeover switch SW to the fixed contact WR sideto apply a voltage of the power supply 10 across the transparentelectrodes 3 and 4 through the terminals 5 and 6 to emit or project,from the glass plate 2 side, the read light RL having a fixed opticalintensity from a light source (not shown).

Namely, since a charge image corresponding to the optical image of theincident light which has reached the photoconductive layer 7 is producedat the boundary surface between the photoconductive layer 7 and theoptical member 8R in the photo-to-photo conversion element into whichwriting of the optical information by the incident light has beenconducted as previously described, an electric field having a strengthdistribution corresponding to the optical image of the incident light isapplied to the optical member (e.g., lithium niobate monocrystal) 9provided so as to have a serial relationship with respect to thephotoconductive layer 7 along with the optical member 8R.

Since the refractive index of the lithium niobate monocrystal 9 changesin accordance with an electric field by the electro-optic effect, therefractive index of the lithium niobate monocrystal 9 provided so as tohave a serial relationship with respect to the photoconductive layer 7along with the optical member 8R under condition where an electric fieldhaving a strength distribution corresponding to the optical image by theincident light is applied to the lithium niobate monocrystal 9, changesin accordance with a charge image produced in correspondence with theoptical image of the incident light, which has reached thephotoconductive layer 7, at the boundary surface between thephotoconductive layer 7 and the optical member 8R in the photo-to-photoconversion element by writing of the optical information of the incidentlight a previously described.

Thus, when the read light RL is projected to the glass plate 2 side, theread light RL which has been projected to the glass plate 2 side goes onthrough the transparent electrode 4, the lithium niobate monocrystal 9and the optical member 8R.

The above-mentioned read light RL is reflected by the optical member 8Rand is then returned to the glass plate 2 side as reflected light. Sincethe refractive index of the lithium niobate monocrystal 9 changes inaccordance with an electric field by the electro-optic effect, thereflected light of the read light RL includes pictorial imageinformation corresponding to the strength distribution of an electricfield applied to the lithium niobate monocrystal 9 by the electro-opticeffect of the lithium niobate monocrystal 9, thus allowing a reproducedoptical image corresponding to the optical image of the incident lightto be produced on the side of the glass plate 2.

In the above-mentioned reproducing operation, the read light which hasbeen projected from the glass plate 2 side, goes on towards thephotoconductive layer 7 via the transparent electrode 4, the lithiumniobate monocrystal 9 and the optical member 8R as previously described.Since the above-mentioned read light is reflected by the optical member8R before it reaches the photoconductive layer 7, it traces an opticalpath including the lithium niobate monocrystal 9, the transparentelectrode 4 and the glass plate 2. For this reason, there is nopossibility that the read light RL reaches the photoconductive layer 7to exert an adverse influence on the charge image of the incident lightwritten thereinto.

As just described above, in accordance with the photo-to-photoconversion element previously proposed, write operation is carried outby allowing the write light WL to be incident from the glass plate 1side, and the reproduction operation is carried out by allowing the readlight RL to be incident to the glass plate 2 side. The method of erasinginformation which has been written into the previously proposedphoto-to-photo conversion element shown in FIG. 2 will now be described.

In the case of erasing information written into the previously proposedphoto-to-photo conversion element shown in FIG. 2, there is employed amethod to switch the movable contact of the changeover switch SW to thefixed contact E side by a switching control signal delivered to theinput terminal 11 for the switching control signal in the changeoverswitch SW connected between the terminals 5 and 6 of the photo-to-photoconversion element, to electrically short-circuit between thetransparent electrodes 3 and 4 so that the transparent electrodes 3 and4 have the same potential to cause an electric field not to be appliedacross both terminals of the photoconductive layer 7, to thereafterallow the erase light EL to be incident from the glass plate 2 side inthe photo-to-photo conversion element.

As described above, the erase ligth EL which has been incident to theglass plate 2 side of the photo-to-photo conversion element reaches thephotoconductive layer 7 via a path including the glass plate 2, thetransparent electrode 4, the lithium niobate monocrystal 9, the opticalmember 8R, and the photoconductive layer 7 to lower the electricresistance value of the photoconductive layer 7, to thereby erase thecharge image formed at the boundary surface between the photoconductivelayer 7 and the optical member 8R.

As understood from the foregoing description, the previously proposedphoto-to-photo conversion element shown in FIG. 2 has an arrangementsuch that the charge image formed at the boundary surface between thephotoconductive layer 7 and the optical member 8R at the time of thewriting operation is erased by the erase light same from the incidentside on which the read light is incident to photo-to-photo conversionelement. Thus, the photo-to-photo conversion element can be easilyapplied to an image pickup device of a structure such that it isrequired to provide the image pickup optical system on the side on whichthe write light WL is incident, and a device of a structure such that itis difficult to provide the incident unit for the erase light on thesame side on which the write light WL is incident. Thus, this previouslyproposed photo-to-photo element can satisfactorily solve the problemswhich afflict the conventional photo-to-photo conversion elementpreviously described with reference to FIG. 1.

Since light incident from the glass plate 1 side generally includeslight in a wavelength region broader than that of visible light, itincludes light in a wavelength region of the erase light EL incidentfrom the glass plate 2 side at the time of erasing.

In the case where light in a wavelength region determined so that it isused as the erase light is included in the light incident from the glassplate 1 side into the photo-to-photo conversion element as describedabove, that light is transmitted through the optical member 8R having awavelength selectivity permitting a light in the wavelength region ofthe read light to be reflected and permitting a light in the wavelengthregion of the erase light to be transmitted, and is then emitted fromthe photo-to-photo conversion element via the lithium niobatemonocrystal 9, the transparent electrode 4, and the glass plate 2.

Thus, in the case where a light in a wavelength region determined sothat it is used as the erase light EL is included in a light incidentfrom the glass plate 1 side into the photo-to-photo conversion elementunder condition where the read or reproduce light RL is incident to theglass plate 2 side for allowing the photo-to-photo conversion element tocarry out the reproducing operation, that light is transmitted throughthe optical member 8R and is emitted from the photo-to-photo conversionelement via the lithium niobate monocrystal 9, the transparent electrode4 and the glass plate 2. For this reason, the reproduced opticalinformation from the photo-to-photo conversion element includes opticalinformation based on light in the wavelength region of the erase lightEL included in the light incident from the glass plate 1 side inaddition to the original reproduced optical information obtained as aresult of the fact that the read or reproduce light RL incident to theglass plate 2 side for allowing the photo-to-photo conversion element tocarry out the reproducing operation is emitted from the photo-to-photoconversion element via the glass plate 2, the lithium niobatemonocrystal 9, the optical member 8R, the lithium niobate monocrystal 9,the transparent electrode 4, and the glass plate 2, resulting in theproblem that correct reproduction operation is not carried out. Afurther problem is as follows. Light having a wavelength longer thanthat of visible light is ordinarily used. However, the fact that lighthaving a wavelength longer than that of visible light is included in thereproduced information is dangerous to the human eye. Thus, it has beenrequired to take countermeasures with respect to this.

SUMMARY OF THE INVENTION

Accordingly, an object of this invention is to provide a photo-to-photoconversion element constituted so that it does not exert an adverseinfluence on the human eye even when an erase light having a wavelengthlonger than that of visible light is used.

In accordance with this invention, there is provided a photo-to-photoconversion element comprising, in an arranged sequence in line: a firstoptical member having a wavelength selectivity permitting to pass awrite light having a first wavelength range, and to block an eraselight; a first transparent electrode; a photoconductive layer; a secondoptical member having a wavelength selectivity permitting to reflect aread light having a second wavelength range and to pass the erase light;a third optical member for electrooptically modulating a light incidentthereto through the second optical member in response to the strengthdistribution of an electric field applied thereto; and a secondtransparent electrode; the erase light having a wavelength outside ofboth the first and second wavelength ranges.

The photo-to-photo conversion element according to this invention isconstituted so that a charge image formed at the boundary surfacebetween the photoconductive layer 7 and the second optical member(having a wavelength selectivity permitting a read light in a wavelengthregion of the visible light to be reflected and permitting light in awavelength region of an erase light having a wavelength longer thanvisible light to be transmitted) 14 at the time of writing is erased bythe erase light same from the incident side on which the read light isincident in the photo-to-photo conversion element into thephoto-to-photo conversion element. Thus, the photo-to-photo conversionelement of this invention can be readily applied to an image pickupdevice of a structure such that it is required to provide an imagepickup optical system on the side of which the write light WL isincident, and a device of a structure such that it is difficult toprovide an incident unit for the erase light on the side on which thewrite light WL is incident. In this instance, the write light in thewavelength region of visible light cannot be transmitted through thesecond optical member, and the second optical member has a wavelengthselectivity permitting the erase light having a wavelength longer thanthat of the write light or the read light in the wavelength region ofvisible light to be transmitted. However, since light having awavelength longer than that of the write light in the wavelength regionof the visible light incident to the glass plate 1 side of an incidentlight does not reach the second optical member due to the presence ofthe first optical member having a wavelength selectivity permittinglight having a wavelength longer than that of the write light in thewavelength region of visible light to be reflected or absorbed, even ifan incident light in a broad wavelength region is incident from theglass plate 1 side into the photo-to-photo conversion element under thecondition where the photo-to-photo conversion element is carrying outread operation, there is no possibility that any adverse influence wouldbe exerted on information read by that light, and that the human eyewould be damaged by light having a wavelength longer than that ofvisible light. Thus, this invention can satisfactorily solve all theproblems encountered with the conventional photo-to-photo conversionelement and the previously proposed photo-to-photo conversion element.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings;

FIG. 1 is a side cross sectional view showing a conventionalphoto-to-photo conversion element;

FIG. 2 is a side cross sectional view showing a photo-to-photoconversion element previously proposed;

FIGS. 3A to 3D are characteristic curves showing the transmission factorof light with respect to the wavelength of a light of an optical memberused in the arrangement of the previously proposed photo-to-photoconversion element shown in FIG. 2, respectively;

FIG. 4 is a side cross sectional view showing an embodiment of aphoto-to-photo conversion element according to this invention;

FIGS. 5 to 8 are characteristic curve diagrams showing the transmissionfactor of light with respect to the wavelength of light of the opticalmember used in the arrangement of the photo-to-photo conversion elementshown in FIG. 1; and

FIG. 9 is a perspective view showing an image pickup device constitutedwith the photo-to-photo conversion element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 4, there is shown an embodiment of a photo-to-photoconversion element of a structure comprising, in a stacked manner, aglass plate 1, a first optical member 13 having a wavelength selectivitypermitting passage of to pass a write light in the wavelength region ofthe visible light as an incident light and permitting to block blockingof an erase light having a wavelength longer than that of the writelight in the wavelength region of the visible light, a first transparentelectrode 3, a photoconductive layer 7, a second optical member 14having a wavelength selectivity permitting reflection of to reflect aread light in the wavelength region of visible light and permitting theerase light to be transmitted, an optical member 9 for changing thestate of light in accordance with a strength distribution of an electricfield applied, a second transparent electrode 4, and a glass plate 2.The first and second transparent electrodes are connected to terminals 5and 6, respectively. In the above structure, the first optical memberneed not be attached to the first transparent electrode 3 but may simplybe placed before the first transparent electrode 3 along a path of thewrite light. Further, the second optical member may be formed as anintegral part of the photoconductive layer.

FIGS. 5 to 8 are characteristic curves showing the transmission factorof light with respect to the wavelength of light in the first and secondoptical members 13 and 14, respectively. The characteristic curvesdesignated by reference numeral 14 in FIGS. 5, 7 and 8 show examples ofthe transmission factor of light with respect to the wavelength of lightin the second optical member 14, respectively. In addition, thecharacteristic curves designated by reference numeral 13 in FIGS. 6, 7and 8 show examples of the transmission factor of light with respect tothe first optical member 13, respectively.

For the first and second optical members 13 and 14, for example, membersconstituted by a dichroic filter of a multilayer film consisting of athin film of SiO₂ and a thin film of TiO₂ may be used, respectively.

For the optical member 9 for changing the state of light according tothe strength distribution of an electric field, for example, an opticalmember constituted by an electro-optic effect crystal such as a lithiumniobate monocrystal or a nematic liquid crystal layer may be used. InFIG. 4, WL, RL and EL denote a write light, a read light and an eraselight, respectively.

In the case of writing optical information into the photo-to-photoconversion element of this invention which has the arrangement shown inFIG. 4, a circuit composed of power supply 10 and changeover switch SWis connected between terminals 5 and 6 of the photo-to-photo conversionelement to allow the movable contact of the changeover switch SW to beswitched to the fixed contact WR side by a switching control signaldelivered t the input terminal 11 for the changeover control signal inthe changeover switch SW. Under this condition, a voltage of the powersupply 10 is applied across the transparent electrodes 3 and 4, to sothat the photoconductive layer 7, the second optical member 14 and theoptical member 9 interposed therebetween, become electrostaticallybiased together. When the write light WL is then caused to be incidentfrom the glass plate 1 side in the photo-to-photo conversion element,writing of the optical information into the photo-to-photo conversionelement will be performed as follows.

The light incident from the glass plate 1 side to the photo-to-photoconversion element at the time of the write operation is a light in abroad wavelength region including light in the wavelength region ofvisible light. The write light WL propagates along an optical pathincluding the glass plate 1, the first optical member 13, the firsttransparent electrode 3, and reaches the photoconductive layer 7. On theother hand, the first optical member 13 provided in the middle of theoptical path from the glass plate 1 to the photoconductive layer 7 has awavelength selectivity as shown in FIGS. 6 to 8, i.e., a wavelengthselectivity permitting light having a wavelength longer than that of thewrite light WL to be reflected or absorbed. Thus, a light having awavelength longer than that of the write light WL cannot reach thephotoconductive layer 7 through the first optical member 13.

The second optical member 14 is constituted as a member having awavelength selectivity permitting the read light in the wavelengthregion of visible light to be reflected and permitting the erase lighthaving a wavelength longer than that of visible light to be passed. Inthis regard, the second optical member 14 may have a bandpasscharacteristic as shown in FIG. 8 to pass only the erase light. And thewrite light WL in the wavelength region of the visible light which hasbeen transmitted through the above-described first optical member 13,i.e., the light which has reached the second optical member 14 via theglass plate 1, the first optical member 13, the first transparentelectrode 3, and the photoconductive layer 7, is also reflected by thesecond optical member 14. Thus the write light WL in the wavelengthregion of the visible light does not pass through the second opticalmember 14.

When the write light WL incident to the photo-to-photo conversionelement is transmitted through the glass plate 1, the first opticalmember 13 and the first transparent electrode 3 to reach thephotoconductive layer 7 as stated above, the electric resistance of thephotoconductive layer 7 along a depth direction thereof varies incorrespondence with an intensity distribution corresponding to anoptical image formed according to the write light which has reached thephotoconductive layer 7. As a result, a charge image (image ofelectrostatic charge) corresponding to the optical image of the writelight WL is produced at the boundary surface between the photoconductivelayer 7 and the second optical member 14.

In order to reproduce the optical information written in the form of thecharge image corresponding to the optical image of the write light WL ina manner stated above, the movable contact of the changeover switch SWis switched to the fixed contact WR side. Under this condition, avoltage of the power supply 10 is applied across the first and secondtransparent electrodes 3 and 4 through the terminals 5 and 6. Then, readlight RL having a fixed light intensity is projected from a light source(not shown) toward the glass plate 2 side in the photo-to-photoconversion element to perform such a reproducing operation.

The charge image corresponding to the optical image produced at theboundary surface between the photoconductive layer 7 and the secondoptical member 14 causes the adjacent optical member (e.g., lithiumniobate monocrystal) 9 to be subject to an electrostatic field of thecharge image corresponding to the optical image formed according to thewhite light.

Since the refractive index of the lithium niobate monocrystal 9 changesin accordance with an electric field due to the electro-optical effect,a refraction characteristic (birefringence characteristic in the case ofthe electro-optical Kerr effect possessed by the optical member) alongthe surface of the lithium niobate monocrystal 9 is changed or modulatedcorrespondingly with the electrostatic field generated by the chargeimage.

Thus, when the read light RL is projected to the glass plate 2 side, theread light RL propagates on through the second transparent electrode 4,the lithium niobate monocrystal 9, and the second optical member 14 bywhich the read light RL is reflected and then returned to the glassplate 2 and exits therefrom as reflected light. As a result, thereflected read light holds an electro-optically modulated pictorialimage information corresponding to the electrostatic field applied tothe lithium niobate monocrystal 9, so that the optical image writteninto the photo-to-photo conversion element PPC by the write light WL isread out by the read light RL and outputted from the side of the glassplate 2 as a reflected light.

As the second optical member 14 is reflective to the read light RLprojected thereto, the read light RL never proceeds beyond the secondoptical member 14. For this reason, there is no possibility that theread light RL will reach the photoconductive layer 7 to cause an adverseeffect on the charge image previously written thereinto.

A method of erasing information which has been written into thephoto-to-photo conversion element of this invention will now bedescribed.

In the case of erasing information, i.e. the charge image written intothe photo-to-photo conversion element of this invention shown in FIG. 4,there is employed a method to switch the movable contact of thechangeover switch SW to the fixed contact E side by a switching controlsignal delivered to the input terminal 11. This causes the transparentelectrodes 3 and 4 to be electrically short-circuited through theterminals 5 and 6, so that the transparent electrodes 3 and 4 read the asame potential, whereby the photoconductive layer 7, the second opticalmember 14, and the optical member 9 interposed between the twoelectrodes become no longer electrostatically biased. Under thiscondition, the erase light EL of uniform intensity and longer wavelengththan that of the read write RL is projected to the glass plate 2 side ofthe photo-to-photo conversion element and reaches the photoconductivelayer 7 via a path including the glass plate 2, the second transparentelectrode 4, the optical member 9, and the second optical member 14,causing an entire body of the photoconductive layer 7 to lower itselectric resistance, so that the charge image formed at the boundarysurface between the photoconductive layer 7 and the second opticalmember 14 becomes dissipated, and no electrostatic charge remains at theboundary. After the erasure, the photo-to-photo conversion element isready to begin a subsequent writing operation as no charge image orelectrostatic charge (DC components) remains.

Since the erase light is projected to the side opposite to the side onwhich the write light WL enters, this invention is easily applicable toan image pickup device of a structure in which an image pickup opticalsystem occupies a space adjoining the side on which the write light WLis incident or to a device of a structure which cannot allow provisionof the erasing system hardware at the side at which the write light WLis incident for some reason. As the second optical member 14 isreflective to the write light WL, the latter does not go beyond thesecond optical member 14 so that it does not disturb the readingoperation if it enters into the photo-to-photo conversion element PPC inthe reading operation. Also, even if an incident light in a broadwavelength region is incident to the photo-to-photo element during theread operation, there is no possibility of a harmful effect of the lighthaving a wavelength longer than that of the visible light to the humaneye looking into the photo-to-photo conversion element PPC.

Regarding the projection of the erase light EL, the erase light EL maynot necessarily be projected from the side to which the read light RL isprojected but may be projected to the photoconductive layer 7 from theopposite side with respect to the second optical member 14 by suitablemeans or along any path directed toward the photoconductive layer 7 fromthe opposite side, which includes a light path along which the writelight WL is incident to the photoconductive layer. When this is thecase, the second optical member 14 does not necessarily pass the eraselight EL but is simply reflective at least to the reading light RL, orreflective to both the reading light RL and the erase light EL.

FIG. 9 is a perspective view of an image pickup device constituted withthe photo-to-photo conversion element PPC of this invention.

In this photo-to-photo conversion element shown in FIG. 9, the surfaceside of the glass plate 1 to which the write light labeled WL in FIG. 4is incident is designated by reference numeral 1 and the surface side ofthe glass plate to which the read light labeled RL and the erase lightlabeled EL in FIG. 4 are incident is designated by reference numeral 2,to clarify the corresponding relationship between the photo-to-photoconversion element PPC in FIG. 4 and that in FIG. 9 and to omit theactual indication of other components in the photo-to-photo conversionelement PPC for brevity of illustration.

Referring to FIG. 9, the image pickup device of this invention includesan imaging lens L for forming an image of an object O, beam splittersBS1 and BS2, a light source PSr for the read light RL (e.g., a flyingspot scanner using a laser light may be used for the light source PSrfor the read light, and such a flying spot scanner system is assumed tobe used for the light source PSr in the following description), a lightsource PSe for the erase light EL, a polarizing plate PLP, and a photodetector PD. In the image pickup device constituted with thephoto-to-photo conversion element PPC illustrated in FIG. 9, an opticalimage of the object is incident from the glass plate 1 side as a writelight to the photo-to-photo conversion element PPC by way of the imaginglens L.

A voltage of the power supply 10 is applied across the first and secondtransparent electrodes 3 and 4 in the photo-to-photo conversion elementPPC set in the write mode and in the read mode through changeover switchSW placed in the state where the movable contact is switched to thefixed contact WR side as shown in FIG. 4. Accordingly, in thephoto-to-photo conversion element constituted so that lightcorresponding to an optical image of the object is incident to the glassplate 1 side through the imaging lens L, as previously described, lightincident from the glass plate 1 side to the photo-to-photo conversionelement at the time of the writing operation is a light having a broadwavelength range including the range of visible light. The light whichpasses along an optical path including the glass plate 1, the firstoptical member 13 and the first transparent electrode 3 reaches thephotoconductive layer 7, which is electrostatically biased as explainedbefore.

As a result, a charge image corresponding to the optical image of theincident light is formed on the boundary surface between thephotoconductive layer 7 and the second optical member 14.

Under the condition that electrostatic bias is applied between the firstand second transparent electrodes 3 and 4, the reading operation of thewritten information is performed from the glass plate 2 side. A coherentread light RL radiated from the light source PSr is projected to a beamsplitter BS1, then is reflected by a beam splitter BS2 toward the glassplate 2 side of the photo-to-photo conversion element PPC. The readlight RL further proceeds to the second transparent electrode 4, theoptical member 9 of which may be lithium niobate monocrystal, and thento the second optical member 14.

The read light RL is reflected by the second optical member 14 andreturns toward the glass plate 2 passing through a body of the opticalmember 9 while being modulated electro-optically due to theelectro-optical effect generated in the optical member 9 by the chargeimage formed in the writing operation. The reflected light thuselectro-optically modulated propagates toward the beam splitter 2,penetrating it to further pass the polarizing plate PLP where theelectro-optically modulated light is converted to an intensity modulatedlight, then is forwarded to the photo detector PD for demodulation,whereby it is reproduced as an electrical signal corresponding to theoptical image previously written.

In the case where a laser beam flying spot scanner is used as the lightsource PSr for the read light RL, the reflected light appearing from theglass plate 2 is a kind of two-dimensional image constituted byelectro-optically modulated flying spot. Accordingly, a video signalcorresponding to the optical image of the object O will be outputtedfrom the photo detector PD.

Writing and reading a series of mutually different picture images at apredetermined interval to produce corresponding frame intervals of avideo signal, it is required to erase each of the formed charge imagesat the predetermined interval, before forming the subsequent chargeimage of a new object to be written.

Such an erasing operation is performed in correspondence with thevertical blanking interval of the video signal to be outputted from thephoto detector PD in such a manner that the movable contact of thechangeover switch SW is operated by a switching control signal deliveredto the input terminal 11, and the erase light EL is intermittentlyprojected at an appropriate timing, both performed in correspondencewith the vertical blanking interval of the video signal.

What is claimed is:
 1. A photo-to-photo conversion element comprising,in an arranged sequence in line;a first optical member for passing awrite light having a first wavelength range; a first transparentelectrode; a photoconductive layer; a second optical member having awavelength selectively reflecting a light in a wavelength band in whicha read light having a second wavelength falls; a third optical memberfor electrooptically modulating a light incident thereto through thesecond optical member in response to the strength distribution of anelectric field applied thereto; and a second transparent electrode.
 2. Aphoto-to-photo conversion element as set forth in claim 1, wherein saidsecond optical member is constituted by a dichroic filter of amultilayer film comprising a thin film of SiO₂ and a thin film of TiO₂.3. A photo-to-photo conversion element as set forth in claim 1, whereinsaid third optical member for electrooptically modulating the lightincident thereto is constituted by an electro-optical effect crystalsuch as a lithium niobate monocrystal or a nematic liquid crystal.